Polyurethane foams are employed in many applications in various industries including for example the refrigeration equipment industry. Illustrative applications include coolers, refrigeration panels, and various types of insulation, among others. In general, low density; rigid polyurethane foams for use in such applications are prepared by reacting an organic isocyanate with a component having at least one isocyanate reactive group, in the presence of a catalyst, blowing agent, and various other additives or reactants. Upon formation of the foam, often the foams are adhered to a desired substrate in order to form a useful product.
Until recently, the blowing agent employed in the production of such foams generally included the use of chlorofluorocarbons (CFC's), optionally with other blowing agents. However, in view of the potential environmental impact of chlorofluorocarbon blowing agents, alternatives are necessary which do not sacrifice the quality of the resulting products. Further, as noted in U.S. Pat. No. 5,430,071 to Green et al., which is hereby incorporated by reference, the flow characteristics of a foaming mixture become extremely important when a blowing agent which instantly volatilizes at atmospheric pressure and temperature is employed. This volatility often causes the foam to froth at the mixing head, which makes it difficult to evenly flow throughout a mold cavity.
Still another important consideration with regard to foams, and particularly low density foams are their surface adhesion characteristics. While many low density rigid foam formulations are known to offer good adhesion characteristics at processing temperatures above approximately 80.degree. F., surface friability tends to become a problem along the foam surface at temperatures below about 80.degree. F. This perceived problem becomes even more pronounced as the processing temperatures fall significantly below 80.degree. F., i.e. between about 65.degree. F. to 75.degree. F. As should be understood by those skilled in the art, surface friability has a dramatic effect on the adhesive characteristics of the foam. In general, the adhesive character of the foam decreases as the friability increases such that the foam will not adhere to the substrate without preapplying adhesives to the substrate itself, which is costly.
Foam molds and equipment operated at temperatures of at least about 80.degree. F. and up to about 110.degree. F. can add significantly to processing costs due to the energy required to heat the mold. Worse yet, a significant segment of the foam manufacturing industry does not have the equipment necessary to heat mold fixtures and substrates above 80.degree. F. as this equipment is often very costly.
Adhesion problems tend to occur with systems using low levels, i.e. less than 10.0%, especially less than 5.0% by weight of the system of HFC or HCFC as blowing agents. While low levels of HCFC and HFC blowing agents have been known to lead to adhesion problems, the use of low level amounts is highly desirable from a cost standpoint. Adhesion problems such as result from surface friability also tend to occur when high water levels are employed, i.e. more than about 3.0 weight percent for the resin and/or 1.5 weight percent based on the foam system. Thus, low water levels should be employed when possible.